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July 2012
Volume 22 Number 2
Combined Voltage and Current
Control Loops Simplify LED Drivers,
High Capacity Battery/Supercap
Chargers & MPPT* Solar Applications
Xin (Shin) Qi
IN THIS ISSUE
pushbutton controller
10
15V, 2.5A monolithic buck-
boost with 95% eficiency
and low noise
17
easy 2-supply current
sharing
20
* Maximum Power Point Tracking
sub-1mm height 24V, 15A
monolithic regulator
26
The rapid expansion of constant-current/constant-voltage (CC-CV)
applications, especially in LED lighting and high capacity battery
and supercapacitor chargers challenges power supply designers to
keep pace with the increasingly complicated interplay of current and
voltage control loops. A switch-mode converter designed speciically
for CC-CV offers a clear advantage,
especially when the supply has limited
power, or its power is allocated
among several competing loads.
deliver 25A at 12V from
inputs to 60V
28
Consider
,
for instance
,
the challenge of charging a
supercapacitor in a minimum amount of time from
a
power-limited
supply. To maintain constant input
power
,
the controlled charging current must decrease
as the output (supercapacitor) voltage increases. The
LT
®
3796 solves the problem of power limited or con-
stant current
/
constant voltage regulation by seamlessly
combining a current regulation loop and two volt-
age regulation loops to control an external N-channel
power switch. The inherent wired-OR behavior of its
three transconductance error ampliiers summed into
the compensation pin
,
V
C
,
ensures that the correct loop
(that is
,
the one closest to regulation) dominates.
(continued on page 4)
The LTC4155 is a monolithic switching battery charger that eficiently delivers
3.5A charge current in a compact PCB footprint. See page 13.
Caption
www.linear.com
Linear in the News
In this issue...
COVER STORY
Combined Voltage and Current Control Loops
Simplify LED Drivers, High Capacity Battery/
Supercap Chargers & MPPT* Solar Applications
Xin (Shin) Qi
1
LINEAR RECEIVES PRESTIGIOUS AWARDS
Over the past several months
,
Linear Technology received several signiicant
awards for products
,
system solutions
,
and for manufacturing quality and delivery.
Here are the highlights:
DESIGN FEATURES
Pushbutton On/Off Controller Includes
Optional Automatic Turn-On When
Handheld Device is Plugged In
Vui Min Ho
Electronic Products 2011 Product of the Year Award: LTC5569 RF Mixer
Electronic Products
honored Linear with the selection of the LTC
®
5569 dual
broadband RF mixer as Product of the Year. From thousands of products
,
the editors of
Electronic Products
selected those they felt are among the
most outstanding. The selection is based on a signiicant advance in technol-
ogy or its application
,
innovation in design
,
or gain in price
/
performance.
10
I
2
C-Controlled Li-Ion Power Management IC with
Integrated Power Devices Charges High Capacity
Batteries from Any 5V Source While Keeping Cool
David Simmons
13
15V, 2.5A Monolithic Buck-Boost DC/DC Converter
with 95% Eficiency and Low Noise Operation
Eddy Wells
The editors stated
,
“In order to make LTE
,
a 4G (fourth-generation) high data-
rate wireless technology attractive to the carrier for deployment
,
base sta-
tion manufacturers worldwide are trying to build a multiband
,
multimode
platform that is easily ield conigured for use in any frequency band and
standard to make the deployment cost attractive. The 26.8
d
B
m
IIP3 LT C5569
dual-broadband RF mixer accomplishes this by combining 26.8
d
B
m
IIP3
,
300
m
W
/
ch
,
and a wide 300MH
z
to 4GH
z
frequency range. So a receiver such
as that based on the LT C5569
,
is capable of working in all of the 700MH
z
,
880MH
z
,
1
.7GH
z
,
1
.8GH
z
,
1
.9GH
z
,
2.4GH
z
,
and 2.6GH
z
bands. In addition
,
within each band
,
the receiver must receive signals that are 60MH
z
wide
,
compared to the previous 20MH
z
wide without sacriicing performance. The
LT C5569 meets both of these requirements with outstanding performance.”
17
Novel Current-Sharing IC Balances
Two Supplies with Ease
Pinkesh Sachdev
20
DESIGN IDEAS
What’s New with LTspice IV?
Gabino Alonso
24
24V, 15A Step-Down Regulator in Sub-1mm Height
Package Pushes Monolithic Performance Limits
Stephanie Dai
26
EN-Genius Network Awards Linear for Products of the Year & the Decade
EN-Genius Network
,
formerly
AnalogZone
,
selected Linear Technology’s
µModule
®
family as Best Product of Our Decade. According to the edi-
tor
,
“The concept of the µModules allows the product designer to
come to the decisions about power management quite late in the
development cycle—simplifying the inal product choice.
Step-Down Converter Delivers 25A at
12V Output from Inputs Up to 60V
Victor Khasiev
28
1.5A Rail-to-Rail Output Synchronous
Step-Down Regulator Adjusts with a Single Resistor
Jeff Zhang
30
back page circuits
32
“Linear has also
extended
the concept to a family of seventeen ‘System in a
Package’ (S
i
P) signal chain receiver modules
,
the irst of which
,
the LTM
®
900
1
,
we reviewed
here
. The S
i
P
s
,
which feature
1
2-/
1
4-/
1
6-bit solutions
,
not
only simplify circuit design for the less than sure RF engineer out there but
they also bypass export regulation control of some individual ADC
s
.
“All-in-all the µModule concept has been a spectacularly successful commer-
cial and technological story for Linear. We are delighted to congratulate them
on their achievement and to recognize them as the Product of Our Decade.”
2 | July 2012 : LT Journal of Analog Innovation
Linear in the news
EN-Genius Network
also selected Linear’s
LTC6946 373MH
z
to 5.79GH
z
Integer-N
synthesizer as Product of the Year for
the Best Integer-N Synthesizer. The pub-
lication commented
,
“The parts are a
welcome addition to Linear’s RF arsenal
and will prove to be equally attractive to
designers who do not have to cope with
large bandwidth issues straight away.”
•
EEPW Editor’s Choice Awards:
Best Analog
Product: LTC6803 battery stack
monitor for hybrid
/
electric vehicles
Best Ampliier: LT
1
999 high voltage
bidirectional current sense monitor
LINEAR’S STRONG PATENT
PORTFOLIO RECOGNIZED
Ocean Tomo announced the leading
companies in patent assets. According
to Ocean Tomo’s ranking
,
Linear
Technology’s patent portfolio received an
IPQ score of
1
23—the highest of any major
analog semiconductor company. Ocean
Tomo claims to rate patent assets objec-
tively based on proven statistical method-
ology. This reinforces the value of Linear’s
growing patent portfolio and the com-
pany’s strong analog intellectual property.
•
Electronic Products China:
Annual
Award: LTM8047/8048 isolated
µModule DC
/
DC converter
Enics Manufacturing Award
Enics
,
one of the largest providers of
electronics manufacturing services
,
named the best suppliers of the year
,
and
honored Linear as the Best Component
Manufacturer at the annual Enics Fair in
Zurich
,
Switzerland.
China Electronics Awards
Several major electronics publications in
China presented Linear with awards:
•
EDN China:
Innovation Award
,
Excellent
Product Award for the LTC4000 high
voltage controller and power manager.
CONFERENCES & EVENTS
Power Systems Show 2012, Tokyo Big Sight,
Tokyo, Japan, July 11–13, Booth 6B-301—
Linear
will showcase power products
,
includ-
ing µModule products and FPGA power
management solutions. More info at
www.
jma.or.jp
/
tf
/
en
11
/electronics
/
index.html
.
•
China Electronic Market:
Editor’s Choice
Award for the Most Competitive
Power Product: LTC4000
EN-GENIUS NETWORK AWARDS LINEAR FOR PRODUCTS OF THE YEAR & THE DECADE
EN-Genius Network
, formerly
AnalogZone
, selected Linear’s µModule products as Best Product of Our Decade. According to the editor, “The concept of the µModules
allows the product designer to come to the decisions about power management quite late in the development cycle—simplifying the inal product choice.”
July 2012 : LT Journal of Analog Innovation | 3
The LT3796’s wide V
IN
range (6V to 100V) and rail-to-rail
(0V to 100V) output current monitoring and regulation allow
it to be used in a wide variety of applications from solar
battery chargers to high power LED lighting systems.
(
LT
3796, continued from page 1)
HIGH POWER LED DRIVER WITH
ROBUST OUTPUT SHORT CIRCUIT
PROTECTION
Figure
1
shows the LT3796 conigured as a
boost converter to drive a 34W LED string
from a wide input range. The LED cur-
rent is derated at low input voltages
to prevent external power components
from overheating. The front-end current
sense ampliier monitors the input cur-
rent by converting the input current to
a voltage signal at the CSOUT pin with
components from exceeding their maxi-
mum rating. If an LED fails open or if
the LED string is removed from the high
power driver
,
the FB constant voltage
loop takes over and regulates the out-
put to 92.5V. The VMODE lag is also
asserted to indicate an OPENLED event.
The additional
,
standalone current sense
ampliier can be conigured for any
number of functions
,
including input cur-
rent limit and input voltage regulation.
The LT3796’s wide V
IN
range (6V to
1
00V) and rail-to-rail (0V to
1
00V) out-
put current monitoring and regulation
allow it to be used in a wide variety of
applications from solar battery char-
gers to high power LED lighting sys-
tems. The ixed switching frequency
,
current-mode architecture results in
stable operation over a wide range of
supply and output voltages. The LT3796
incorporates a high side current sense
,
enabling its use in boost
,
buck
,
buck-
boost or SEPIC and lyback topologies.
The LT3796 includes short-circuit pro-
tection independent of the LED cur-
rent sense. The short-circuit protection
feature prevents the development of
excessive switching currents and pro-
tects the power components. The
protection threshold (375
m
V
,
typ) is
designed to be 50% higher than the
default LED current sense threshold.
R
R
6
5
V
I
R
=• •
CSOUT
IN
SNS
1
The resistor network at the FB
1
pin
provides OPENLED protection
,
which
limits the output voltage and prevents
the ISP pin
,
ISN pin and several external
V
IN
R
SNS1
50m
L1 22µH
D1
I
IN
Figure 1. A 34W LED driver with robust
output short-circuit protection.
9V TO 60V
100V (TRANSIENT)
C2
2.2µF
×4
100V
C
1
2
.2µF
×3
R7
1M
R5
2k
R1
1M
R3
499k
V
IN
V
S
CSP
CSN
LTspice IV
circuits.linear.com/558
R8
13.7k
R2
118k
EN/UVLO
GATE
M1
R4
97.6k
SENSE
OPTIONAL INPUT
CURRENT REPORTING
R
SNS
15m
CTRL
UP TO
400mA
GND
CSOUT
CSOUT
C3
10nF
R6
40.2k
FB1
PWM
PWM
LT3796
ISP
SYNC
SYNC
Q1
R
LED
620m
LED CURRENT REPORTING
ISMON
ISN
C4
0.1µF
INTV
CC
R10
100k
R9
100k
TG
M2
FAULT
FAULT
INTV
CC
INTV
CC
VMODE
VMODE
C5
4.7µF
D2
V
REF
FB2
V
C
RT
SS
85V LED
R
T
31.6k
250kHz
M1: INFINEON BCS160N10NS3-G
M2: VISHAY SILICONIX Si7113DN
L1: COILTRONICS DR127-220
D1: DIODES INC PDS5100
D2: VISHAY ES1C
Q1: ZETEX FMMT589
LED: CREE XLAMP XR-E
R
C
10k
R11 OPTIONAL
FOR FAULT LATCHOFF
R11 402k (OPT)
C6
0.1µF
C
C
10nF
4 | July 2012 : LT Journal of Analog Innovation
design features
The LT3796 solves the problem of power limited,
or constant-current/constant-voltage regulation
by seamlessly combining a current regulation
loop and two voltage regulation loops to control
an external N-channel power switch.
If there is no resistor between the SS pin
and V
REF
pin
,
the converter enters hiccup
mode and periodically retries as shown
in the Figure 2. If a resistor is placed
between V
REF
and SS pin to hold SS pin
higher than 0.2V during LED short
,
then
the LT3796 enters latchoff mode with
GATE pin low and TG pin high
,
as shown
in Figure 3. To exit latchoff mode
,
the
EN
/
UVLO pin must be toggled low to high.
SS
2V/DIV
SS
2V/DIV
LED+
50V/DIV
LED+
50V/DIV
FAULT
10V/DIV
FAULT
10V/DIV
I
M2
1A/DIV
I
M2
1A/DIV
5ms/DIV
5ms/DIV
LED DRIVER WITH HIGH PWM
DIMMING RATIO
Using an input referred LED string
allows the LT3796 to act as a buck
mode controller as shown in Figure 4.
The
1
MH
z
operating frequency enables
a high PWM dimming ratio. The
OPENLED regulation voltage is set to
Figure 2. Short LED protection: hiccup mode
(without R11 in Figure 1)
Figure 3. Short LED protection: latchoff mode
(with R11 in Figure 1)
Once the LED overcurrent is detected
,
the
GATE pin drives to GND to stop switch-
ing
,
the TG pin is pulled high to discon-
nect the LED array from the power
path and the FAULT pin is asserted. The
Schottky diode D2 is added to protect
the drain of PMOS M2 from swing-
ing well below ground when shorting
to ground through a long cable. The
PNP helper Q
1
is included to further limit
the transient short-circuit current.
. V
R
R
3
6
R
R
5
4
125
•• +
1
V
IN
through the independent current sense
ampliier at CSP
,
CSN and CSOUT pins.
During the PWM off phase
,
the LT3796
disables all internal loads to the V
C
pin
16V TO 36V
R
LED
250m
M2
LED
+
1A
Figure 4. A buck
mode LED driver
with 3000:1 PWM
dimming ratio
R3
100k
R4
100k
R1
1M
8V
LED
C2
10µF
×4
25V
V
IN
V
S
ISP
ISN
TG
EN/UVLO
CSP
R5 1M
R2
100k
V
REF
CSN
Figure 5. 3000:1 PWM dimming ratio of the circuit in
Figure 4 at V
IN
= 24V and PWM = 100Hz
CTRL
CSOUT
FB1
L1
10µH
R6
124k
LT3796
PWM
PWM
D1
FB2
ISMON
C1
2.2µF
×2
50V
M1
GATE
LED CURRENT REPORTING
PWM
5V/DIV
C
5
0.1µF
SENSE
INTV
CC
R
SNS
33m
R8
100k
R9
100k
I
L
1A/DIV
GND
FAULT
FAULT
VMODE
VMODE
INTV
CC
INTV
CC
C3
4.7µF
V
C
RT
SYNC
SS
I
L
1A/DIV
R
C
10k
R
T
6.65k
1MHz
C
4
0.1µF
M1: VISHAY SILICONIX Si73430DV
M2: VISHAY SILICONIX Si7113DN
D1: ZETEX ZLLS2000TA
L1: WÜRTH 744066100
LED: CREE XLAMP XM-L
C
C
4.7nF
1µs/DIV
July 2012 : LT Journal of Analog Innovation | 5
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